MRI provides a unique non-invasive imaging method for discriminating the main components of human disease pathology. As a result, MRI is one of the most widely used diagnostic imaging tools in today's hospitals throughout the world. A typical MRI system includes a main magnet to generate a uniform DC magnetic field, three gradient coils to generate linear and orthogonal magnetic field gradients, a transmitting and receiving radio frequency (RF) antenna to generate imaging pulses and receive the resulting RF emissions, and an operator interface and control station. For human imaging the magnet is mainly superconducting in nature and has a cylindrical shape, although at the present time open "C" arm magnet geometries are also used for imaging the human body. For higher strength magnetic fields (0.5 T and higher), the superconducting magnet is used to generate a highly uniform static magnetic field with a clear bore diameter of 90 cm or larger for human patient access.
Gradient coils are electromagnetic coils capable of generating linearly varying and axially directed static magnetic fields along the three spatial directions (x,y,z) of a Cartesian coordinate system. The function of each one of the three orthogonal gradients is to encode the spatial information as a frequency or phase variation. In general, higher gradient strengths of 40-100 mT/m with faster rise times of 40-150 .mu.sec at full strength are required for faster imaging techniques. Standard methods for production of linear magnetic field gradients in MRI systems consist of driving discrete coils with a current source of limited voltage. The discrete coils are wound in a bunched or distributed fashion on an electrically insulating hollow light cylinder coil-form.
A digital radio frequency transmitter transmits radio frequency pulses or pulse packets to a whole body RF coil to deliver RF pulse into the examination region. The RF pulses are used to excite, prepare, saturate, invert, refocus, or manipulate the resultant bulk magnetization due to ensemble average of the magnetic moment of a specific nuclear spin such as proton in selected portions of the examination region. For whole body applications, the resonance signals are commonly picked up by the same whole body RF coil. For other more regional focused applications, the signals are often picked up by local coils placed at the vicinity of the examination region other than the whole body coil. Alternatively, a receive-only coil can be used to receive resonance signals induced by the body RF coil. For example, in human head imaging, an insertable head coil can be inserted surrounding a human brain at the isocenter of the magnet used for receiving the RF signal. Conventional RF coils for MRI application are the birdcage coil, single loop surface coil and surface array coil.
In MRI, the resultant radio-frequency signals, which are spatially encoded, are picked-up by the receiver RF coil, amplified and then demodulated/digitized by a receiver. A sequence controller controls or schedules the timing sequence of the three orthogonal gradients, RF pulse waveforms, frequency offset, RF phase, data sampling window of the receiver, as well as other events such as triggering to generate a variety of MRI sequences, such as spin echo imaging, gradient echo imaging, fast spin echo imaging, and echo planar imaging. An image reconstruction processor sorts the spatially encoded image data according to the order in which they are received and transforms the data to form the final MR image.
More specifically for RF antenna, a simple conductor loop interrupted with some capacitors of proper values can serve as a local RF antenna that is capable of transmitting RF signal to its vicinity and detecting minute RF signal from its vicinity with a relatively high signal-to-noise ratio (SNR). Such a coil can only transmit and detect one component of the magnetization signal during M imaging, and often is referred as a linear (RF) coil. Since the detectable magnetization for MRI is a two dimensional vector, to improve the SNR and B.sub.1 uniformity, a quadrature version which receives two orthogonal components simultaneously was then introduced for the same geometry configuration.